ASN said: “The performance of these inspections will require shutdown of the reactors concerned.”

According to analyses performed by EDF at ASN’s request since 2015, some of the channel heads of the steam generators made by Areva Creusot Forge or Japan Casting & Forging Corporation (JCFC) have been found to contain a significant carbon concentration zone.

The carbon concentration could lead to lower than expected mechanical properties.

Out of 18 reactors which are equipped with steam generators, 12 reactors featuring channel heads manufactured by JCFC are liable to contain a particularly high carbon concentration.

ASN said: “EDF has provided ASN with data aiming to demonstrate the operating safety of the 12 reactors concerned.

“Further to the analysis carried out with the support of IRSN and after technical discussions with EDF, ASN has concluded that additional inspections should be carried out within three months, without waiting for the scheduled refuelling outage of these reactors.”

The nuclear authority said that the inspections are aimed at establishing that each of the channel heads concerned is in conformity with the file transmitted by EDF.

In April last year, ASN found an anomaly in the composition of the steel in certain zones of the vessel closure head and the vessel bottom head of the Flamanville EPR reactor.